1996 Chevy Truck

Tiny
JLAFOLLETTE
  • MEMBER
  • 1996 CHEVROLET TRUCK
  • V8
  • 2WD
  • AUTOMATIC
  • 12,000 MILES
In the midst of an engine swap, we lost spark. Going through the basic ignition system test, there is a short to ground in tachometer input circuit. This grounding is not in the wiring, only coming from the VCM #8 terminal. The computer is new. Is there a short in other inputs causing this circuit to be grounded?
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Monday, July 27th, 2009 AT 12:55 PM

12 Replies

Tiny
DOCFIXIT
  • EXPERT
Hi
ECM terminal 8 is camshaft sensor return should have from 1 to 4.5 v. Possible faulty canshaft sensor located in distributor. Are you using the old dist. Or did engine come with one?
Let me know
Thanks for donate
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Monday, July 27th, 2009 AT 3:31 PM
Tiny
JLAFOLLETTE
  • MEMBER
I described the problem improperly. After testing for 1-4V AC on the module terminal B, no volts(.016) I tested for short to ground. The IC circuit was grounded at the VCM terminal 9, white wire. When I unplugged it the grounded circuit went away, and I determined the computer was the source of the ground. The computer has been replaced. Disregard the tach circuit and #8 terminal mentioned earlier.
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Tuesday, July 28th, 2009 AT 2:02 PM
Tiny
DOCFIXIT
  • EXPERT
With new ECM does it run?
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Tuesday, July 28th, 2009 AT 2:08 PM
Tiny
JLAFOLLETTE
  • MEMBER
The long story is that we installed a new long block. As old school mechanics, we missed swapping the crank reluctor so we pulled the timing cover off and installed. It ran but rough idle. We bought the OBDII scanner to set timing, not it, pulled intake for leaks, pulled cover again to check timing chain, checked lift to make sure it didn't have a bumpy cam, put it together- no spark. Found a ground off, but still no spark. Accidently left the key on so thought maybe some component damage. Distributer was new on install, bought crank sensor. Chk'd coil, module, no codes. Got talked into a VCM, no spark. Have run the flow chart over and over, changed meters, testers, there is only.016 V AC on module terminal B. The IC circuit is grounded, VCM terminal #8 white wire, but if you unplug it, it is open. The ground is coming from the computer. Any ideas?
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Tuesday, July 28th, 2009 AT 4:09 PM
Tiny
DOCFIXIT
  • EXPERT
Take a look at this schematic see if it will help trace down fault Coil driver maybe causing ground. Also what is engine size? so you put long block swaped out other parts and old engine had spark?


http://www.2carpros.com/forum/automotive_pictures/198357_Graphic_6.jpg

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Wednesday, July 29th, 2009 AT 9:02 AM
Tiny
JLAFOLLETTE
  • MEMBER
I appreciate your responses, Docfixit. The schematic you provided is similar to what I have from Mitchell. The truck is a 1996 C20 w/ 7.4 VIN J. It died from a distributer drive gear failure. The long block came from an Ebay engine rebuilder. It had spark when we got done, but ran rough. We attributed the rough running on the first start-up to dist timing, but when curing that failed, we dug in and ended up without spark. Then we were thinking, what did we forget to hook up or what ground was left off or what wire is pinched, but we have exhausted ourselves on all of that. Now we have been chasing down the possibility of a component failure from leaving the ignition on overnight. I'm striking out. I am assuming a crank or cam sensor would produce a code. The crank sensor is new, the dist new on new motor install. I had a local part store test the module twice, they say it is good. Going through the No Spark test provided in another topic on this website (1996 Tahoe no spark) I went through the flowchart down to testing for 1-4 AC volts on the B terminal of the module. That's where I stall, as it does not have this voltage and the circuit is grounded. The grounded IC circuit doesn't exist unless it is plugged in to the VCM. Probing the VCM #9 terminal (IC circuit) with the C3 plug disconnected shows the computer is where the grounding comes from. I'll finish off with a nice donation if you stay with me on this.
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Wednesday, July 29th, 2009 AT 11:27 AM
Tiny
DOCFIXIT
  • EXPERT
Put a test light on terminal B of coil driver white wire crank does test light flash If yes go to coil white/black wire crank flash if no coil driver faulty if yes then coil.
Let me know
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Wednesday, July 29th, 2009 AT 11:39 AM
Tiny
JLAFOLLETTE
  • MEMBER
After beating crank sensor on the floor, my spark returned. Replaced the new sensor, truck runs but set a p1345. Reinstalled dist. Many times, had timing at many locations, but can't get it down to 0 or 2 deg, 14 adv was the best, but can't do that again. It hangs out at 22 or 24, will adv a few with throttle, I have seen 31 once. Anybody?
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Thursday, August 13th, 2009 AT 3:06 PM
Tiny
JLAFOLLETTE
  • MEMBER
As a supplement to the above question, I have followed the procedures described in manuals and Alldata. Com on dist installation. My aftermarket dist does not have an "8" marking as the target for rotor location on final placement, but I have had the rotor pointiing at #1 contact and the engine atTDC on compression stroke, and moved it 1 tooth each direction as well as rotated the dist housing to try to pick up 0 degree timing, with code cleared. It wouldn't drop down or even respond to changes. It isn't frozen on 24 degrees, as it will increase in timing with throttle application a few degrees. I've read some chatter about crank relearning, but there hasn't been any reference to it in instructions for dist replacement.
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Thursday, August 13th, 2009 AT 4:57 PM
Tiny
DOCFIXIT
  • EXPERT
Assume you goy new crank sensor after beating on floor? Your aftermarket Dist. is it replica of GM? can old dist be used? Here are some pics of dist placement. Align pin hole in gear with paint mark on shaft. Refer to old dist. align rotot as in fig. 9 install should end up like fig. 10.


http://www.2carpros.com/forum/automotive_pictures/198357_Graphic_57.jpg


http://www.2carpros.com/forum/automotive_pictures/198357_Grafic_3_11.jpg


first fig.8 then 9 then 10


http://www.2carpros.com/forum/automotive_pictures/198357_Grafic_4_2.jpg

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Friday, August 14th, 2009 AT 8:46 AM
Tiny
JLAFOLLETTE
  • MEMBER
Yes, replaced Crank sensor. Old dist had bad bushing and drive gear, that's what took out the old motor. There won't be a white line to align to, as that would be if you were re-installing original dist. The new NAPA dist is similar enough, meaning that ultimately my target is to have the rotor pointing at the beginning of the flat part of the casting which is actually #1 cyl. I get that part, that's old school. I'm confident in the dist placement, my problem is that it still shows 22-24 deg adv on the scanner, and adjustment or even moving a tooth doesn't affect it. Two teeth makes it crossfire.
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Friday, August 14th, 2009 AT 9:16 AM
Tiny
DOCFIXIT
  • EXPERT
Does RPM fluctate at idle? Have you checked Knock sensors? There are two each side block they should ohm at 3300 to 4500 ohms. When taping on block near sensor does idle change? Is it noted on scanner? Also have you checked with timing light? If confident in dist then crank reluctor is in question any warpage a nick in blade?
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Friday, August 14th, 2009 AT 12:12 PM

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